This invention relates to communication satellite systems and more particularly relates to bandwidth allocation techniques for such systems.
Current satellite communication systems rely on bulk bandwidth allocation. The current Single Carrier per Channel (SCPC) and Time Division Multiplexing (TDM) systems allocate a fixed bandwidth for the duration of a user's session. This requires that the user actively initiate and terminate a data transfer session. For this reason, the bandwidth utilization can vary drastically based on the type of activity being performed. For rapid transfer of data the bandwidth has to be over allocated and under utilized.
Communication over satellites is characterized by large propagation delays for a geostationary satellite, the one-way delay between a user and the satellite or the two-way delay between two users is a minimum of 250 milliseconds. Yet, users must share a common resource: the uplink bandwidth. Just as communications between users is subject to large delays, so also is the communication between users and the bandwidth manager (BWM) responsible for allocating uplink bandwidth among users.
The propagation delay makes it extremely difficult for the BWM to be responsive to the bandwidth needs of the system's users. Traditionally, designers have either allocated fixed amounts of bandwidth to each user for relatively long periods (allocating each user a circuit, in effect), so that each user would have adequate bandwidth under any circumstance (a committed information rate (CIR) approach), or they have insisted that each user request every small amount of bandwidth that they need, so that users would get only the bandwidth that they could actually use (the bandwidth-on-demand approach).
The CIR approach is wasteful of bandwidth in several ways. It ignores the fluctuation in use of bandwidth caused by users downloading or uploading and then pausing between operations. It ignores the asymmetry in consumption of user uplink bandwidth between downloads and uploads when the user is downloading a file, he needs to uplink only an occasional acknowledgement back to the sender, but when he is uploading a file, he will uplink large amounts of data. It ignores the variations in bandwidth utilization that occur over the course of a single upload or download establishment and termination of a file transfer use very small amounts of bandwidth compared with the bandwidth used to transfer the file. However, the CIR approach provides very good quality of service (QoS), because the user always has as much bandwidth as he could have expected.
The bandwidth-on-demand approach, on the other hand, is very efficient in its allocation of bandwidth. In this approach, users request bandwidth only when they have data to send, and they request only as much bandwidth as they need to send their backlogged data. Thus, almost all the allocated bandwidth is actually used. However, the bandwidth on-demand approach can drastically cut throughput, and the QoS as perceived by the user can be terrible. The system operates in fits and starts, as users have backlogged data, request a limited amoutn of short-term bandwidth, wait through the delay to get bandwidth, send data, and then repeat the process. While there are methods that partially ameliorate the problems with the bandwidth-on-demand approach, its service quality still remains poor.
This invention addresses these problems and provides a solution.